Fused bath electrolytic production of ferrochromium



Nov. 17, 1942. c. E. DOLBEAR 2,302,604

I FUSED BATH ELECTROLYTIC PRODUCTION OF FERROCHROMIUM Fi led Nov 15 1959 INVENTOR.

71 45 ATTORNEYS.

Patented Nov. 17, 1942 FUSED BATH ELECTROLYTIC PRODUCTION OF FERROCHROMIUM Clinton E. Dolbear, Alameda, Calif., assignor to Alexander W. Reid, Clinton E. Dolbear, and Lloyd Veitch, as trustees Application November 15, 1939, Serial No. 304,525

2 Claims. (Cl. 204-71) This invention relates to the electrolytic production of metals from their ores and compounds.

In my copending application Serial No. 198,607,

filed March 28, 1938, I have disclosed a process of electrolytically reducing metallic ores and metallic compounds in accordance with which such ores or compounds as conduct electricity at high temperatures are consolidated to form solid articles which are used as anodes in an electrolytic cell comprising a fused-bath and a fused metal cathode. Under the influence of the electric current the metallic compound or ore of which the anode is composed is decomposed so that its metallic constituent enters the electrolyte and is deposited at the liquid metal cathode. In the practice of that invention the fused cathode may be the metal which is to be recovered from the ore, or, if desired, it may be a metal which is to be alloyed with that of the ore, for instance to produce the so-called ferroalloy metals, but in either event the metal is deposited in fused, or molten, form.

The invention disclosed and claimed in my aforesaid copending application is satisfactorily operative to achieve its objective. However, its practice entails the preliminary step of consolidating or bonding the ore to convert it to electrode form for use as anodesnnthe process, and

it entails the use of quite high-temperatures, particularly in cases such as the production of pure chromium, ferro-chromium, and other high melting point metals'and alloys, with the operating and economic disadvan ges attendant upon the use of such high temp ature in electrolytic cell operation.

pounds which become conductors of electricity at elevated temperatures may be directly reduced electrolytically by connecting such ore or compound as an anode in an electric cell provided with a fused bath and operated at a temperature below the melting point of the metal extracted from the ore so that it is deposited in solid form at the cathode of the cell. In the practice of I It is among the objects of the present invento the accompanying drawing in which Fig. 1 is a vertical sectional view through an electrolytic cell illustrative of the preferred practice of the invention; and Fig. 2 a cross-sectional view of the cell taken on line lI-H, Fig. 1.

The present invention is predicated upon my discovery that metallic ores and metallic com- 55 anode to supply ions of, or containing, the metal this invention there may be used anodes of consolidated ore or compound as described in my aforesaid application, but for most purposes it is preferred to use an anode of the ore or compound in unconsolidated; loose form. The cell construction and operation are simplified and cheapened through the ability to use temperatures far below those necessary in the practice of the process described in my copending application.

In the practice of the present invention, therefore, a mass of the ore, eithenconsolidated or in fragmentary, or granular, form is connected in the 'electric circuit of the electrolytic cell to act as an anode. The cathode is preferably solid, most suitably metallic, and in any event where it is desired to produce carbon-free metal or alloy it is of non-carbonaceous nature. The

fused electrolyte used in the practice of the in-- vention most suitably comprises a salt bath, and

for most purposes it is preferred to use salts or mixtures of salts which melt, or fuse, at relatively low temperatures. Halides, or mixtures of halides, preferably chlorides, are preferred for most purposes, examples being sodium, potassium and calcium chlorides and mixtures of two or more of them.

By operation of the cell at temperatures below the melting point of the metal to be produced, the metal is deposited in solid form at the oathode, commonly in the physical state termed sponge metal in the art. The bath is selected with reference to the metal to be recovered, the

criteria being the use of a fused bath'whieh under the influence of the electric current causes the metallic constituent of the anode to enter the bath and be ionized for deposition of the metal at thecathode, and which is operated at a temperature below the melting point of 'the metal deposited at the cathode.

This invention differs from those electrolytic processes in which the metal-containing material is initially dissolved in the fused electrolyte. In the practice of this invention the ore acts as anode and is electro-decomposed by deposition f the anion, which reacts with the metal in the for transport to and deposition at the cathode, thus automatically maintaining the concentration of the metal in the electrolyte by decomposition of the ore anode.

Where ores and similar substances containing gang'ue or other waste matter are used in the practice of the invention there may be a tendency for the gangue matter to contaminate the metal deposited, for which reason it is preferred in such instances to provide the cell with a porous diaphragm which prevents the gangue matter from reaching, the cathode. I have found that for many purposes diaphragms made from periclase ar fused silica are suitable. Such porous diaphragms are, of course, well known in the electrolytic art, and other types may be used. At low operating temperatures the solid gangue tends to float to the surface so that it may be skimmed off as need be. If the gangue tends to remain in the cell,-for example due to operation at higher temperatures, it can be collected in various ways, as by setting up currents in the anode chamber to concentrate the gangue at one side, where it may be moved.

In the practice of the invention the electrolysis is conducted at a potential and current density such as to produce the desired result and, of course, the passage of the current will maintain the electrolyte in heated condition. Inasmuch as the anode may be, and suitably is, constituted of ore in loose and unconsolidated granular, or particulate, form, and inasmuch as the invention is applicable to the production of a variety of metals from divers ores and compounds, and to the use of various fused electrolytes, it is not possible to state limiting ranges of voltage and current density applicable to all materials and all conditions of subdivision in which they may be treated. The proper current conditions for use in the practire of the process are easily determined for any particular case, however, as will be understood by those skilled in the art. The anode current density should be such as to avoid, if possible the so-called anode effect, and the character of the metallic deposit is dependent upon the cathode current density. Of primary importance is the potential at which the cell is operated, but this is readily determined for any given case. The potential must be such as to effect deposition of the desired metal at the cathode and attack of ore at the anode by the anion liberated thereon. As an example, if chrome ore is being treated in a sodium chloride bath chlorine deposited at the chrome ore anode may attack the ore to form chromium chloride which dissolves in the bath and is electrolyzed with deposition of chromium at the cathode and of chlorine at the anode, for solubilizing of a further amount of chromium. However, if the temperature is not high enough to permit rapid attack of the ore chlorine may be liberated from the cell. An empirical mode of attaining the proper potential in the case of the example just cited is to increase it until sodium appears at the cathode, and then to reduce the voltage applied until sodium is not liberated; and when operating properly oxygen but no chlorine escapes from the anode chamber.

The anode efl'ect referred to is the phenomenon commonly encountered in the electrolysis of a fused salt bath which manifests itself by a sudden decrease in the amperage and an increase in the voltage across the cell. When this occurs the electrolyte apparently does not wet the anode,

which is surrounded by a gas film through which cent of bentonite.

the current passes in small arcs, often accompanied by a hissing noise or a glow.

Although the cell is operated at a temperature such that the metal deposited at the cathode is in solid or sponge form, it is desirable to operate at as high a temperature as possible because the specific resistance of the cell decreases with increasing temperature. On the other hand, the temperature should not be so high as to cause loss of valuable constituents. For instance, in electrolyzing chromite ore as anode in a chloride bath the temperature should be low enough to avoid substantial vaporization of chromium chloride from the electrolyte.

The invention may be described further with reference to the accompanying drawing and to its application to the production of chromium by the electrolysis of chromite ore. As shown in Fig. 1, there is provided an electrolytic cell comprising walls and hearth of suitable refractory l adapted to withstand the temperatures and the materials to which it is exposed, for instance, magnesite refractory bricks. The refractory may be encased in a metallic shell 2. The cell is provided with a porous diaphragm 3 which is resistant to the temperatures and chemical influences of the cell, and which divides it into an anode chamber 4 and a cathode chamber 5. In the bottom of anode chamber 4 is a mass 6 of loose unconsolidated particles or granules of chromite or chromite concentrates. This mass may be connected to act as anode in any suitable manner, but preferably the bottom of the anode chamber is provided with a lining, or hearth, that receives the anode material and is electrically conductive, at least at the temperature at which the cell is operated. I havediscovered that a particularly desirable way of doing this, at least for the electrolysis of chromite ore, is to first place in the bottom of the cell a layer 1 of a mixture of chromite concentrates with about 2.5 per it becomes converted to a very hard and dense body that becomes electrically conductive at elevated temperatures and thus serves to conduct current to the loose anode mass of ore.

Embedded within layer I are one or more metallic tubes 8 which project from the sides of the cell for connection to the positive pole of a suitable source of direct current. When the layer 1 of bonded chromite is heated to a temperature at which it conducts the electric current satisfactorily and a current is supplied to tubes 8, mass 6 of chromite is supplied with current so that it acts as anode in the cell.

The cell is provided also with a fused electrolyte 9, and metal fromanode B is deposited at a cathode III which, in the embodiment shown, projects downwardly into the electrolyte in the cathode chamber and is, of course, connected to the negative pole of the source of direct current. For many purposes the cathode may be made of Nichrome."

. In the embodiment shown current is conveniently supplied to the cell from a direct current generator I l whose positive side is connected by a lead I2 to tubes 8, and the negative side by a lead l3 to cathode Hi. Should it be desirable or necessary, heat may be supplied to the anode to bring it to operating temperature by resistors II and Ma mounted in tubes 8. Suitable resistors are those formed from silicon carbide, such as those sold under the trade-mark Globar. In the cell shown one end of resistor H is connected by a lead 15 to a suitable current source, and at When this mixture is heated its other end it is connected by a lead Hi to one end of resistor l4a, the other end of which is connected by a conductor I! to the other side of the current source. In this manner the heating elements may be operated independently of the supply of electrolyzing current.

Suitably the cell is provided also with -a gastight cover l8 having a tube l9 leading from vide a body 6 of suitable thickness which is to act as anode in the electrolysis. If need'be, heating current is then passed through resistors I4 and Ma tobring the temperature up to the necessary point, whereupon current is passed from generator II to the electrodes to initiate electrolysis. In the treatment of chromite there will the metal deposited at cathode III will consist of a mixture or alloy of iron and chromium. With the current density adjusted properly, oxygen will be evolved at anode 6 which will rise through the electrolyte in anodechamber 4 and be drawn off through conduit 19. From time to time further amounts of chromite in granular, or par- .ticulate, condition are introduced through feed .tube v2!). The gangue matter of the ore is maintained within anode chamber 4 by diaphragm 3 so that the metal collected in the cathode chamber is not contaminated thereby, but is pure and carbon-free.

The invention may be illustrated further with reference to its actual application to the electrolysis of chromite ore. In one run there was used a cell having a lining of magnesite brick and a porouh diaphragm separating the cell into anode and cathode chambers. The bottom of the anode chamber, of about 28 square inches area, was provided with a hard and dense layer of chromite concentrates formed as described hereinabove in which there was embedded a 1- inch diameter Nichrome" tube which projected beyond the sides of the furnace and which was connected to the positive side'of a supply 'of direct current. Mounted within the tube was a Globar unit in an electric circuit. About 300 grams of Montana chromite or e were added to the anode chamber to form the anode for electrolysis. he cathode consisted of a Nichrome plate. The electrolyte consisted of a mixture of sodium and potassium chlorides in approximately equal parts by weight. Heat was supplied also by /4-ir1ch diameter Nichrome rods formed into coils with their ends parallel to each other and normal to the coil axis to permit insertion in the cell. They were connected to a transformer of low potential and high current capacity, each coil receiving about 225 amperes. The heat from these rods was used to melt the electrolyte salts.

When the electrolyte was molten and the anode suiiiciently hot these rods were withdrawn, current was supplied to the electrodes, and there were added to the anode chamber 226 grams of chromic chloride, while 453 grams of chromic chloride were added to the cathode chamber.

be formed both iron and chromium ions, so that .being red hot throughout.

formed on the surface of the electrolyte in the The chromic chloride was added, of course, to promptly supply chromium ions for immediate electrolysis and thus to avoid or reduce the induction period which would have otherwise been necessary to bring the cell to operating equilibrium conditions.

At the start the current was at 12.8 volts and 150 amperes, and the cell resistance was 0.085 ohm. During the time it took the cell to reach a stable condition there were added 1004 grams of Turkish chromite to the anode chamber to replenish and maintain the anode mass of loose ore.

'The resistance of the cell decreased steadily until it became stable at 0.0252 ohm, in which condition the current was steady at 7.7 volts and 305 amperes. This condition continued until the end of the run. During runs -"of this nature I have made the interesting observation that if the current supplied to the cell is cut off, a current contin'ues to flow in the same direction and at a potential of about 2.5 volts; the reason for this result is not clearly understood.

1 At the end of the'run the furnace was evenly and thoroughly hot, the magnesite brick lining No gangue crust anode chamber, and the accumulation of gangue matter did not appear to interfere in any way 'with'the operation of the cell.

At no time during the run was any considerable amount of sodium liberated at the cathode, and at no time was there any odor of chlorine about the cell. The metal collected from the cathode chamber consisted of an alloy or a mixture of approximately per cent of chromium and 10 percent of iron in carbon-free form.

Where ores such as chromite (FeO.Cr2Oa) are treated in accordance with the invention the material produced will normally be a mixture or alloy of the metals contained in the ore, althoughmy work indicateslithat by suitable adjustment. of operating conditions, such as voltage, it maybe possible to effect separation of a desired metal, such as chromium, in substantially pure form. Also, I have observed of the products produced by electrolysis of chromite tin accordance with this invention .that they consist apparently of a mechanical mixture of iron and chromium codeposited as such rather than as an alloy, so that separation of substantially pure chromium should be' possible by simple means, as by magnetic methods.

Pure metals may, of. course, be recovered by the treatment of simple ores, as for example by the electrolysis of pyrolusite to produce commercially pure and carbon-free manganese, or by the electrolysis of commercially pure oxides or other compounds. For instance,chromium oxide may be used as anode to produce chromium free from carbon and iron. From what has been said it will be understood that the word compound used in the claims contemplates not only pure, or commercially pure metallic compounds, but also metallic ores, and that the word metal includes not only pure metals but also alloys.

Various modifications from what has been described specifically may be practiced without departing from the spirit of the invention. For instance, the cathode may .be arranged other than as shown in the drawing, or a plurality of cathodes may be used. Similarly, it is not essential that heat to start operation be supplied as described or shown. For example, hot molten electrolyte may be poured into the cell to supply the electrolyte ready for electrolysis and by its heat to bring the anode up to operating temperature. Furthermore, although the detailed description based upon the drawing involves the use of loose, unconsolidated ore it will be realized from what has been said that the ore may be consolidated to form coherent anodes, for example as described in my aforesaid application. Other modifications will appear to those skilled in the art.

According to the provisions of the patent statutes I have explained the principle and method of practicing my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. That method of producing Ierrochromium which comprises passing .an electric current Irom an anode of loose, unconsolidated chromite through a fused halide electrolyte comprising at least one halide of the group sodium, potassium and calcium, to a solid cathode, adjusting the voltage and amperage of said current to cause solution of iron and chromium from said anode and deposition of solid ferrochromium at said cathode and to maintain said electrolyte molten at a temperature below the melting point 01 said ferrochromium, and adding fresh loose and unconsolidated ore to said anode to maintain the anode as it is consumed.

2. A process according to claim 1, said halide electrolyte being a chloride electrolyte.

3. A process according to claim 1 in which the electrolyte is a chloride electrolyte, the anode and cathode are separated by a porous diaphragm, and the anode current density is adjusted to cause liberation of oxygen at the anode.

CLINTON E. DOLBEAR. 

